Lightning locating system

ABSTRACT

A lightning detection system for detecting and locating an initial discharge of an initial leader stroke of a lightning flash. An initial lightning discharge produces a pulse that can be used to accurately detect lightning, and more particularly, the location of the initial lightning discharge. In one embodiment, at least three sensors detect and determine the location of the first pulses from initial lightning discharges using time difference of arrival information of the pulses at each of the three sensors. In another embodiment, a single sensor is used to determine the range of an initial lightning discharge from the amplitude of a corresponding initial detected pulse, and to determine its direction from a crossed loop antenna. An alternative embodiment of a single sensor system determines a distance of a lightning event from a peak amplitude value derived from a pulse amplitude distribution. In a further embodiment, a lightning detection system provides enhanced lightning location by incorporating weather data from a weather radar with detected lightning information.

RELATED CASE INFORMATION

The present application claims benefit of U.S. Provisional applicationSer. No. 60/001,540, filed Jul. 26, 1995.

FIELD OF THE INVENTION

The invention relates generally to lightning warning systems, and moreparticularly to an early lightning warning system for detecting andprocessing initial lightning discharges.

BACKGROUND OF THE INVENTION

Lightning includes electrical discharges within a cloud, intracloud (IC)discharges, and cloud to ground (CG) discharges (total lightning).Lightning occurs when electrical fields within a cloud intensify asparticles of opposite polarity collect at differing regions within thecloud. Typically, the electric field forms as a result of strongupdrafts which carry monopolar positive charge aloft leaving negativespace charge in a central or lower part of the cloud. Precipitation anddowndrafts can also transport negative space charge downward. Theupdrafts and downdrafts further contribute to the electrification of thecloud particles. Lightning generally occurs near the location of theseintense updrafts and downdrafts.

Lightning begins with an initial electrical breakdown (pulse) followedby leader channels from which a series of channel branches grow within acloud forming a comprehensive branch channel structure. For IClightning, the channel structure remains within the cloud. A CGdischarge occurs when one or more branches extend from a cloud to theground. The leader channel propagates in steps to the ground. When theleader channel is about 100 meters from the ground, a streamerpropagates up from the ground to meet the stepped leader. When the twomeet, a continuous channel of ionized air is formed from the cloud tothe ground. At this point a large current flows from the ground to thecloud which is known as a return stroke.

Typical lightning detection systems, such as the Lightning Location andprotection (LLP) system used in the National Lightning Detection Network(NLDN), operate to detect CG return strokes. Generally, the returnstroke associated with a CG discharge is many times larger than for ICdischarges. Thus, such a system typically will not detect IC lightning.Also, the system assigns a location to the discharge corresponding to aposition on the ground and provides no information with respect to thestroke origin which may be tens of miles distant.

A further disadvantage of prior single sensor systems is susceptibilityto RF noise. Since Very Low Frequency (VLF) signals are targeted fordetection, systems for detecting CG return strokes can provide a generalbearing of a storm using well known crossed loop technology, but aresubject to gross errors with respect to distance. In particular, singlesensor VLF systems determine the distance of a lightning stroke from thestoke intensity, but stroke intensities can vary by two or three ordersof magnitude. Thus it will be appreciated that these VLF systems do nothave the capability to accurately determine lightning range from asingle observation station.

Another disadvantage of known systems stems from display limitationswith respect to lightning position. More particularly, a single dot on adisplay typically represents a complete lightning flash. However, alightning flash can extend for tens of kilometers from an initialleader. Thus a display of dots may provide a general area containinglightning discharges, but does not provide an accurate representation ofthe location of the source of the atmospheric disturbance. The source ofthe lighting is generally the area presenting the most severe aviationhazards, such as hail, icing, turbulence and microbursts.

A still further drawback of some systems is that in operation, thesystems detect and process energy from many parts of the lightningchannels of lightning strokes and the multitude of pulses from eachstroke, thus requiring a tremendous processing capability. Such systemsare complex and expensive.

With respect to known VLF single station lightning detection systems,there are considerable limitations associated therewith due to theinherent variation in lightning stroke discharge amplitudes. Forexample, a lightning channel structure includes a tremendous horizontaland vertical span radiating energy throughout. This producespolarization errors for azimuth and distance determination. Lightningdischarges vary in intensity as much as three orders of magnitude, thusprecluding accurate distance determination based on detected dischargeintensity. Also, IC and CG discharges have different characteristics.These factors contribute to the problems of fabricating an accuratelightning detection system including the ability to distinguish betweenIC and CG lightning.

Often during thunderstorms, intense downdrafts, known as microbursts,follow lightning producing updrafts. Microbursts pose a threat toaircraft, especially immediately after take off and prior to landingwhere an aircraft is especially vulnerable. A further danger to aircraftresults as a microburst approaches ground level and air flowshorizontally creating a wind shear region possibly resulting in stallingthe aircrafts and losing lift. In fact several hundreds of deaths haveoccurred in airplane crashes over the past few decades due to intensedowndrafts, or microbursts and resulting wind shear. Since lightninggenerally begins near the locations of intense updrafts and downdrafts,the early detection of microbursts is critical in averting suchdisasters. This situation has been partially addressed by the FederalAviation Administration (FAA) which has responded by situating weatherradars, such as Terminal Doppler Weather Radars (TDWR), at various majorairports across the United States. These radars measure the radialvelocity of raindrops towards and away from the radar and infer airmotions therefrom. However, despite the considerable cost, in theneighborhood of several millions of dollars, Doppler weather radars havelimitations. For instance, if rainfall is vertical and the radar isscanning near the horizon, no radial velocity is detected, thus notdetecting a possible downdraft. Doppler radars operate to detect outflowair having rain drops therein. Further disadvantages of weather radarsare slow volume scans, for example up to three minutes to obtain onepicture, performance degradations due to ground clutter, and significantcost.

Known lightning detection systems do not provide a way to determinepotential microburst locations since there is no known correlationbetween CG discharges and microbursts. It will be appreciated by oneskilled in the art that a VLF system detecting signals havingwavelengths on the same scale as lightning channels is not well adaptedfor microburst prediction which requires defining lightning in scales ofhundreds of meters. A VLF system detects a CG return stroke emitting VLFenergy having a wavelength in the order of 10 kilometers, but canrarely, if at all, detect the shorter stepped structure of IC lightningrich in HF and VHF radiation from which microbursts can be predicted.

Other technologies are currently being developed and exist to detectwindshear conditions, such as laser, Infra Red (IR) and Doppler radarbased systems. While those technologies may be successful inascertaining microbursts that have already developed, it is unlikelythat event prediction will be attainable. Hazardous weather warningswould be in the vicinity of a few minutes, or seconds, and thus possiblynot sufficient for an aircraft to avoid the danger.

In view of the above, a low cost total lightning warning system isdesired which detects all lightning discharges and provides an earlywarning of lightning, accurate location including direction and range ofthe source of lightning, lightning type, and the location of potentialmicrobursts.

SUMMARY OF THE INVENTION

The present invention overcomes the aforesaid and other disadvantages byproviding a lightning detection system for detecting initial lightningdischarges which allows lightning to be detected prior to cloud toground strokes. The initial discharges provide information used todetermine lightning location and possibly hazardous weather conditionssuch as intense updrafts and downdrafts or microbursts.

In a first embodiment, a lightning detection system includes at leastthree sensors disposed in locations known to a master station which iscoupled to each of the sensors. The sensors detect a pulse produced byan initial lightning discharge of an initial lightning stroke. At eachsensor, a time of pulse arrival is recorded which information is sent tothe master station for processing. The master station uses the timedifference of arrival of the pulse at the three sensors to determine thelocation in two dimensions of the origin of the initial discharge. Fouror more sensors provides a location in three dimensions.

In another embodiment, a lightning detection system includes a singlesensor for determining the location of an initial lightning discharge(RF pulse) The initial pulses are monopolar, either positive or negativeand about one microsecond or less in duration. The sensor includes anE-field antenna and a crossed loop antenna. A distance for the initialdischarge is determined from the amplitude of the detected pulse whichis constant for initial pulses. Since the initial discharge is verticalwith respect to ground and its associated RF pulse has the sameamplitude for all first pulses, the range can be accurately determinedsince the amplitude is known to fall off as a function of distance. Inconjunction with the E-field antenna, the crossed loop antenna providesa direction for the initial lightning pulse. An altitude for the initialpulse can be assigned base on the polarity of the corresponding pulse,since initial IC discharges are negative and CG discharges are positive.

A further embodiment of a single sensor lightning detection systemdetects a series of microsecond or shorter pulses emitted by a lightningstroke and determines a peak or average value from a distribution of thepulse amplitudes. This value can be used to determine the distance ofthe lightning event.

In an alternative embodiment, a lightning detection system includes aweather radar and a lightning detector for providing weather radar typedisplay including lightning information.

BRIEF DESCRIPTION OF THE DRAWINGS

Other features and advantages of the invention will be apparent from thefollowing detailed description of the invention in conjunction with theaccompanying drawings in which:

FIG. 1 is a pictorial representation of lightning forming in a cloud;

FIG. 2 is graphical representation of a typical cloud to groundlightning flash showing a change in electrostatic potential over time;

FIG. 3 is a graphical representation of an electromagnetic signature ofthe cloud to ground lightning flash of FIG. 2;

FIG. 4 is graphical representation of a typical intracloud lightningflash showing a change in electrostatic potential over time;

FIG. 5 is a graphical representation of an electromagnetic signature ofthe intracloud lightning flash of FIG. 4;

FIG. 6 is a block diagram of a first embodiment of a lightning detectionsystem according to the present invention;

FIG. 7 is a block diagram of a sensor forming a part of the lightningdetection system of FIG. 6;

FIG. 8 is a block diagram of a master station forming a part of thelightning detection system of FIG. 6;

FIG. 9 is pictorial representation of an exemplary display for thelightning detection system of FIG. 6;

FIG. 10 is a block diagram of a further embodiment of a lightningdetection system according to the present invention;

FIG. 11 is a circuit diagram of an E-field antenna and amplifier forminga part of the lightning detection system of FIG. 10;

FIG. 12 is a circuit diagram of a loop antenna and amplifier forming apart of the lightning detection system of FIG. 10;

FIG. 13A is a circuit diagram of an E-field pulse detection circuitforming a part of the lightning detection system of FIG. 10;

FIG. 13B is a circuit diagram of a logic circuit forming a part of thelightning detection system of FIG. 10;

FIG. 14 is a circuit diagram of pulse measurement circuit forming a partof the lightning detection system of FIG. 10;

FIGS. 15A and 15B are flow diagrams illustrating process stepsassociated with a processor forming a part of the lightning detectionsystem of FIG. 10;

FIG. 16 is an exemplary distribution of pulse amplitudes correspondingto a lightning stroke; and

FIG. 17 is a block diagram of an alternative embodiment of a lightningdetection system including a weather radar according to the presentinvention.

DETAILED DESCRIPTION OF THE INVENTION

As shown in FIG. 1, lightning is a result of an electrical potentialgradient in a cloud 10 formed as an updraft 12 in a cloud convectivecycle carries positive charge to an upper portion 14 of the cloud.Positive charge remains in a lower portion 16 of the cloud so that amiddle layer 18 having a majority of negative charge is formed betweenthe upper and lower portions 14,16 of the cloud. Large electricalvertical potential gradients (which control the directions of theinitial pulse) are thus developed near the boundaries between thenegative and positive space charge at the top 24 and bottom 26 of thelayer 18 leading up to an initial vertical lightning discharge throughnon-ionized air forming a channel. The initial vertical dischargeprovides initial ionization of the air forming a channel of air.

An initial IC discharge 22 originates at a first intersection 24 of theupper portion 14 and the middle portion 18 of the cloud 10. Generally,the intersection is about 9 km in altitude for a mature thunderstorm,but may initially be at 6 or 7 km. An initial CG discharge 20 originatesat a second intersection 26 of the lower portion 16 and the middleportion 18 of the cloud 10. Typically, the altitude of the initial CGdischarge is about 5 km. The initial IC and CG lightning discharges22,20 are substantially vertical. It will be appreciated that theinitial CG discharge 20 and the initial IC discharge 22 will be ofopposite polarity. The initial IC and CG discharges form a portion of aninitial leader stroke from which a series of channel branches growforming a comprehensive branch channel structure 28. A series of strokesflows through the channel structure, which is known as a flash. A flashis generally observed as flickering as lightning channels emit light.

The initial IC and CG discharges are virtually identical except forpolarity. However, although IC discharges are present throughout athunderstorm, IC discharges almost always precede the first CG strokereaching the ground by tens or more minutes. Thus IC detection iscritical to providing an early warning of thunderstorms, prior to CGlightning strokes reaching the ground. In fact, IC lightning detectionis far more accurate in detecting thunderstorms, since somethunderstorms do not produce any CG lightning and others produce only afew CG flashes.

As shown in FIG. 2, an electrostatic signature of a CG flash 32 includesan initial leader stroke signal 34, an initial return stroke 36,followed by subsequent strokes 38 and subsequent return strokes 40. TheCG flash 32 thus includes a plurality of strokes. As shown, a series ofpositive steps, seen as return stroke signals 36,40, represent CGdischarges. The initial leader stroke 34 includes an initial discharge42 at the very beginning of the initial leader stroke. The CG flash 32stretches over several hundred milliseconds, and is detectable by a"slow antenna" sampling as low as 0.1 Hz for measuring the electrostaticpotential difference over time.

FIG. 3 illustrates the detected RF energy from the CG flash 32 of FIG.2. Corresponding to the initial discharge 42 of the initial leaderstroke 34, there is an initial pulse 44. The initial pulse 44 has acharacteristic amplitude 46, energy and duration that is relativelyconstant. The initial return stroke 36 produces a corresponding initialreturn stroke pulse 48. The subsequent return stroke 40 correspond toreturn stroke pulses 50. The return stroke pulses 50 are followed byQ-bursts 52, represented as a series of pulses having an amplitude lowerthan that of the return stroke pulses.

FIGS. 4 and 5 illustrate an electromagnetic signature and correspondingresulting pulses of an IC flash 54. As can be seen, the IC flash 54includes negative steps which are less pronounced in comparison with theCG flash 32. A recoil streamer 56, similar to but generally smaller thana CG return stroke, produces a corresponding pulse 58. An initialdischarge 60 of an initial IC leader stroke 61 corresponds to an initialRF pulse 62 having a characteristic amplitude 64, energy and durationsubstantially matching that of the CG initial pulse, except in polarity.It will be appreciated that detecting and processing initial IC and CGpulses provides an advance in the art over known lightning detectionsystems.

In a first embodiment as shown in FIGS. 6-8, a lightning detectionsystem includes a plurality of sensors for detecting RF energy fromlightning discharges. It is believed, but without limitation thereto,that RF energy transmitted by virtually all initial lightning dischargeswithin a cloud are substantially vertical with respect to ground. It isfurther believed that RF pulses produced from almost all initiallightning discharges, IC and CG, are substantially similar with respectto energy, amplitude, and duration, regardless of the eventual energy ofa stroke, or whether the discharge eventually reaches ground. Thus aninitial lightning discharge and corresponding pulse can be the basis forearly and accurate lightning detection and location.

A first embodiment of a lightning detection system 100 includes fivesensors 102 and a master station 104 for collecting and processing datafrom the sensors. The sensors 102 are configured in spaced apartrelationship wherein the exact location of each sensor is known to themaster station 104. In an exemplary embodiment, sensors 102 are placedin a cross-configuration with one sensor centrally located within thecross and disposed within a twenty-five mile radius. A (Globalpositioning System) GPS unit associated with each sensor can provide alocation accurate to about fifty feet, but in an illustrativeimplementation, the accuracy of each sensor with respect to latitude andlongitude is known to within 10 feet. Each of the sensors 102 isconnected to the master station 104 via telephone line or by a RS-232serial computer link 106. An RS-232 link is adapted for a sensor in thevicinity of the master station.

Different configurations of the sensors are possible in alternativeembodiments. For a three-dimensional system a minimum of four sensors isnecessary and for a two-dimensional system, three sensors are required.In one embodiment, at least one sensor is located in the generalvicinity of an airport. Alternatively, one or more sensors can belocated on aircraft and provide information to a master station and toother aircraft. The sensors can be placed hundreds of miles apart withsufficient sensor sensitivity providing improved accuracy of lightninglocation. As a further alternative, sensors can be placed at aircraftwingtips. Although the wingspan distance is limited, autocorrelationtechniques reduce error to an acceptable level. In another embodiment,lightning data is provided to unmanned lightning warning stationstypically located near smaller airports.

Referring now to the block diagram of FIG. 7, each sensor 102 includes alightning antenna 110 in communication with a pre-amplifier 112 foramplifying a signal provided by the antenna. In an exemplary embodiment,the antenna 110 detects RF energy in a range from 150 kHz to 50 MHz. Abipolar comparator 114 is coupled to the pre-amplifier 112 for detectingpulses having characteristics within predetermined ranges correspondingto an initial lightning discharge. A detected pulse generally has aduration of about a microsecond or less with a rise time of about onehundred nanoseconds and a fall time somewhat longer. The comparator isbipolar in that positive and negative pulses are detected. A GPS unit116, including a GPS antenna 118, provides location, time, and dateinformation to a sensor processor 120 via a bi-directional GPS datasignal 122 which also allows the sensor processor to communicate withthe GPS unit. The GPS unit 116 provides a one second pulse signal 124 toa counter 126 which is coupled to the processor 120 by a count valuesignal 128. The comparator 114 is connected to the counter 126 andsensor processor 120 by an event signal 130. In an exemplary embodiment,the event signal 130 is connected as an interrupt to the sensorprocessor 120. Coupled to the sensor processor 120 is memory 132 forproviding event data storage capability. A modem 134 and serial link 136are coupled to the sensor processor 120 to provide stored eventinformation to the master station 104.

The GPS unit 116 a provides a frame of reference with respect to time.It will be appreciated that other methods can be used to provide a timereference. The GPS unit 116 can also provide a location for the sensors(within about 50 feet), but greater accuracy is afforded by surveyedlatitude and longitude measurements (within 10 feet). The pulse signal124 from the GPS unit 116 is active once per second and is operative toreset the counter 126. The counter 126 is incremented by a crystaloscillator 138 having a frequency of about 40 MHz, thus providing aclock period of about 25 ns.

In operation, the comparator 114 activates the event signal 130 upondetecting a pulse having an amplitude greater than a predeterminedamplitude. The active event signal 130 stops the counter 126 fromincrementing and provides an interrupt to the sensor processor 120. Thesensor processor 120 reads the value in the counter 126 via the countvalue signal 128 which corresponds to the time of the detected pulse.The counter 126 provides a "time stamp" for the event. Moreparticularly, the time of the detected pulse is known by the value inthe counter 126 which corresponds to the time since the last one secondpulse from the GPS unit 116. The sensor processor 120 then prohibits anyfurther event signals to the counter 126 for a predetermined time, 0.5to 1.5 seconds for example. This allows the GPS unit 116 to reset thecounter 126. Each initial discharge pulse (event) detected by thecomparator 114 has assigned thereto event information including GPSlocation, date, and time, and time from the most recent one second GPStime update (counter value), and polarity. The pulse information is heldin memory 132 under control of the sensor processor 120. The minimumstorage capacity of the sensor is 100 lightning events. The stored eventinformation in the memory 132 is retrieved by the master station 104 viamodem or serial link. As shown in phantom, in a further embodiment, thesensor 102 can also include a bipolar peak detector 140 coupled inbetween the preamplifier 112 and sensor processor 120 for providing peakvoltage information for the event.

In exemplary embodiments discussed herein, references to an initialpulse associated with an initial lightning leader may also include aseries of subsequent pulses. For example a predetermined number ofpulses, such as 5 or 10 pulses, from the first part of the initialleader may be processed to provide verification of a first initialpulse. Or processing may include an average over several pulses toimprove accuracy. Furthermore, a time sample may be on the order of amillisecond or more. Typically, prior to an initial discharge there is aperiod, typically at least about one hundred milliseconds, of relativequiet with respect to detected RF radiation. Exemplary embodiments canprovide for periods of about a one-tenth second to 1 second before asystem is ready to process another initial pulse or event. However, thepresent invention includes alternative embodiments that continuallyprocess lightning events.

As shown in FIG. 8, the master station 104 includes a control processor142 and a display 144. Event information associated with an initiallightning discharge pulse is retrieved by the control processor 142 forprocessing and providing the processed information to the display 144for viewing by a user. The master station 104 polls each of the sensors102 for event data and requests stored event data if any is present. Themaster station 104 processes event data acquired from the sensors 102 todetermine a location of an event in three dimensions. The controlprocessor 142 first groups events according to the time of arrival (TOA)of the event. An algorithm may be used to determine the location,including altitude, of the initial lighting discharge corresponding tothe detected pulse (event) from the time difference of arrival (TDOA) ofthe detected pulses. Least mean square algorithms are used determine themost probable location if there is more than one solution which occursif more than four sensors are used. Processed lightning event data isdisplayed on the display 144. The control processor 142 in conjunctionwith the display 144 provides information with respect to lightningstroke rate and the rate of change with respect to the lightning rate.The stroke rate and rate of change are available for IC and CG lightningindividually, and for the total lightning rate, i.e., IC and CGlightning.

FIG. 9 is an exemplary display 145 of collected initial lightningdischarge information. As can be seen, the display 145 shows areas ofvarying lightning intensity, such as low hazard 146, medium hazard 148,and high hazard 150. It will be appreciated that other displayembodiments can include differentiation by color, or dimension. Forexample, a three-dimensional display can provide altitude information.In an alternative embodiment, a single or multiple displays can provideinformation relating to two or more height ranges.

The high resolution mapping and intensity determination of IC lightningprovide the lightning detection system the ability to predict microburstevents. Microbursts can be detected from IC lightning as discussedbelow, and disclosed in U.S. Pat. No. 4,996,473 to Markson et al.,incorporated herein by reference. A microburst downdraft is generallyabout 1-2 km in diameter, within the resolution for the lightningdetection system of the present invention. Since lightning is caused inpart by updrafts in a convective cycle, it follows that along theperimeter of the updraft, IC lightning discharges begin. It is knownthat a microburst is preceded by a an updraft, thus allowing microburstprediction based on IC lightning having certain characteristics, and inparticular a sharp increase in the lightning discharge rate in a cellsized region. In particular, there is a sharp monotonic increase inlightning rate which maximizes about 5 to 10 minutes before a maximumoutflow velocity associated with a microburst. For example, when the IClightning rate increases to a rate of 5 strokes per minute, or more,followed by rate decay, there is a greater than 90% chance that amicroburst will occur.

Pulse information also provides a measure of storm intensity derivedfrom strokes per minute. In typical lightning detection systems, stormintensity cannot be measured because they cannot detect or locate mostIC flashes. A lightning detection system according to the presentinvention provides storm intensity as a function of flashes per unittime.

Cloud height is also an important factor in a determination of stormintensity. Lightning is proportional to the 5^(th) power of cloudheight. It follows that a 10% increase in cloud height, results in a 60%increase in lightning rate. The increase in cloud height may not bedetected by radar. As air velocities in convective clouds areproportional to cloud height, vertical air motion (updraft velocities),are closely related to lightning rates. An advantage of accuratethundercloud location includes the ability to allow a plane to fly underor around thunderclouds, thereby minimizing air traffic interruptions.The cost, delay and frustration associated with weather delays is wellknown. Thus, it will be appreciated that a lightning detection systemproviding accurate lightning discharge altitudes provides a significantadvantage.

Lightning information is used to provide hazardous weather warnings toaircraft. Dangers for aircraft near a thunder cloud include hail, icing,turbulence, microbursts and being hit by lightning. The amplitude ratioof CG to IC lightning is about 10:1 at about 10 kHz, while above 1.5MHz, the ratio is about 1:1. While VHF signal energy attenuates at arate of one over frequency, VHF lightning radiation can be detected at adistance of at least 200 km. At higher frequencies the signal issmaller, as is the noise, so that the S/N ratio remains about the same.In other embodiments, a lightning detection system includesdifferentiation of lightning discharges by frequency ranges, providing aplurality of frequency bins which would also be available forprocessing.

A lightning detection system can be in data communication with aircraftvia Mode S and other data links. Air-ground data link communication isexpected to be available across the country in the next few years.Lightning data is provided to aircraft enabling a pilot to avoidhazardous weather conditions, yet minimize unnecessary rerouting.

As an alternative to TDOA processing, a further embodiment includes theutilization of direction finders to determine the location of an initialdischarge pulse. Direction finders are known in the lightning detectionart. In another embodiment, at least one direction finder is disposed onan aircraft. Information from the direction finder is communicated to amaster station via a Mode S transponder or another data link. A furtherembodiment includes lightning sensors disposed on a plurality ofaircraft to form an airborne lightning detection network, constantlyupdating a lightning database.

FIGS. 10-14 illustrate a second embodiment of a lightning detectionsystem having a single sensor. Prior single sensor lightning detectionsystems have been unable to accurately provide range data for lightningdue in large part to lightning variations in intensity and direction.The lightning detection system of the present invention takes advantageof the known characteristics of an initial pulse of a lightningdischarge to determine the location of the discharge source and to inferthe altitude from differentiating between an IC and CG pulse. Inparticular, the initial lightning discharge is vertical with respect toground and has a known amplitude that falls off as a function ofdistance. The initial pulse also has a known duration of about onemicrosecond or less. Furthermore, IC and CG discharges aredistinguishable by polarity. The single sensor system is adapted forplacement on an airplane or ground.

An exemplary single sensor lightning detection system 200 includes an Efield antenna 202 and a crossed loop antenna 204, including a referenceloop 206 and a quadrature loop 208, for detecting respective electricfield and magnetic field components of a traveling wave radiated by aninitial lightning discharge. The E-field antenna 202 is coupled to anE-field amplifier and filter 210, the reference loop antenna 206 iscoupled to a reference amplifier and filter 212, and the quadrature loopantenna 208 is coupled to a quadrature amplifier and filter 214. Each ofthe respective amplifier filters 210,212,214 is coupled to a respectiveE-field, reference, and quadrature pulse measurement circuit 216,218,220each providing analog input signals to an A/D converter 222. The filtersfor the magnetic and electric signal paths have the same characteristicsin an exemplary embodiment. The A/D converter 222 provides an outputsignal to a processor 224. The E-field amplifier and filter 210 is alsocoupled to a signal filter 226, which in turn provides an output to apulse detector circuit 228. The pulse detector circuit 228 is coupled toa logic circuit 230 which provides a timing signal to each of the pulsemeasurement circuits 216,218,220. The logic circuit 230 is coupled tothe processor 224. The processor 224 provides a serial data outputsignal 232.

FIG. 11 illustrates one embodiment of an E-field antenna 202 andamplifier circuit 210. The antenna 202 is flat plate in a firstembodiment having a diameter of about 3 cm with a capacity of about 20pfd. Other embodiments for the antenna include a thin wire and a sphere.The amplifier circuit 210 includes an amplifier 240 and an RC network242 coupled in a feedback relationship with the amplifier. The antenna202 is coupled to a positive input of the amplifier 240. The sensitivityof the circuit is determined by ratio of the shunt capacity of theamplifier inputs to the antenna capacity. The shunt resistance and shuntcapacity determine the low frequency cutoff of the E-field channel andthe high frequency cutoff is determined by the RC network 242capacitance.

FIG. 12 illustrates an exemplary embodiment of a reference loop antenna206 and amplifier circuit 212. In an exemplary embodiment, thequadrature loop antenna 208 and amplifier circuit 214 are equivalent tothe reference circuits. The reference loop antenna 206 includes a 6 cmlong antenna having a time constant about the same as the E-field flatplate antenna 202. Two coil direction finders are well known in the art.It will appreciated that many other alternatives are possible. The loopantenna 206 includes ferrite rods having a multi layer winding and iselectrostatically shielded. The loop antenna 206 is connected across apositive input 250 and a negative input 252 of a wide band differentialop amp 254. An output 256 of the amplifier is coupled to the negativeinput 252 in a feedback relationship through a resistor 256. The lowfrequency cutoff of the circuit 212 is determined by the coil terminalresistance and the high frequency cutoff is determined by the firstorder resonant frequency.

FIG. 13A shows one embodiment of the filter 226 and bipolar pulsedetection circuit 228 of FIG. 10. The output of the E-field amplifierand filter circuit 210 provides a signal to the pulse detection circuit228 which compares the voltage of a potential pulse to a predeterminedvoltage threshold. The pulse detection circuit 228 includes a firstamplifier 260 providing a pulse voltage level signal 262 to a firstcomparator 264 having an output signal 266 and to a second comparator268 having an output signal 270. A first reference amplifier 272provides a positive threshold voltage signal 274 to the secondcomparator 268, and a second reference amplifier 276 provides a negativethreshold voltage signal 278 to the first comparator 264.

In operation, when the pulse voltage level signal 262 provides apositive voltage level to the second comparator 268 greater than thepositive threshold voltage signal 274, the second comparator outputsignal 270 is active. When the pulse voltage level signal 262 provides anegative voltage level to the first comparator 264 greater than thenegative threshold voltage signal 278, the first comparator outputsignal 266 is active. It will be appreciated that other embodiments forthe described circuit are possible.

FIG. 13B shows an exemplary embodiment of the logic circuit 230 of FIG.10. The logic circuit 230 includes a first OR gate 280 coupled to aone-shot multivibrator 282. The first OR gate 280 receives the first andsecond comparator output signals 266,270. A latch 284 comprising firstand second NOR gates 286,288 is coupled to the multivibrator 282. Asecond OR gate 290 is coupled to the latch 284 and provides a pulsedetected signal 292 to the processor 224 of FIG. 10. The latch 284receives a reset signal 294 from the processor 224. Also coupled to themultivibrator 282 is an inverter 296 providing a hold/detect signal 298.Also coupled to the latch 284 is a buffer 299 providing a measure/resetsignal 297. In operation, when either of the first or second comparatoroutput signals 266,270 (FIG. 13A) is active, the first OR gate 280triggers the one-shot 282, which sets the latch 284 activating the pulsedetect signal 292 to the processor 224 (FIG. 10). The triggered one-shot282 activates the hold/detect signal 298. After the processor 224detects the active pulse detect signal 292, the processor causes thereset signal 294 to become active after a predetermined amount of time,thus resetting the latch 284 and the measure/reset signal 297.

FIG. 14 is an illustrative embodiment of the pulse measurement circuits216,218,220 of FIG. 10. The pulse measurement circuit coupled to each ofthe amplifier and filter circuits 210,212,214 is equivalent in anexemplary embodiment. The pulse measurement circuit 216 includes anamplifier 300 having a gain of about 26 and a delay circuit 302providing a delay of about 400 ns coupled to an output of the amplifier300. The delay allows switching transients to settle. The circuit 216includes a peak detection circuit 304 which includes parallel paths forprocessing positive and negative pulses. At an output of the delay line302 and prior to the pulse detection circuit 304, a first switch 306 iscoupled. The first switch 306 activated by the hold/detect signal 298 ofFIG. 13B, enabling pulse information to propagate to the peak detectioncircuit 304. The peak detection circuit 304 includes first and secondamplifiers 308,310, wherein for a voltage greater than a predeterminedlevel, a capacitor 312 charges to present a voltage at the capacitor tothe second amplifier 310. A second switch 314 is coupled to an input ofthe second amplifier 310 in parallel with the capacitor 312 fordischarging the capacitor thereby effectively turning off the circuitwhen the measure/reset signal 297 of FIG. 13B is not active, i.e., thereis no pulse to be measured. It will be appreciated that peak detectioncircuits are well known to one skilled in the art. In an exemplaryembodiment, the peak detection circuit 304 has a response time of about50 ns. The peak detection circuit 304 provides a positive peak valuesignal 316 and a negative peak value signal 318 to the A/D converter 222of FIG. 10. The A/D converter 222 receives positive and negative peakvalues signals for each of the respective E-field, reference, andquadrature circuit paths.

FIGS. 15A-B show a flow diagram 400 of one embodiment of the processingsteps associated with event data sent to the processor 224 of FIG. 10.In step 402, the processor initializes peripheral equipment 402 and thenwaits at step 404 for a trigger. Upon receiving a trigger, the processorreads values from the A/D converter in step 406. In step 408, in orderto screen out nonlightning events an E/H ratio computed from the A/Dconverter values is compared to a predetermined value. The E/H value fora lightning discharge is about 377 and an exemplary acceptable range ofvalues is within the acceptable lightning discharge E/H ratio. Afterfinding that the E/H ratio for the pulse is within acceptable limits, arange and bearing of the event is computed in step 410. The bearing iscomputed from the A/D channels as follows:

    ______________________________________                                        Channel         Data                                                          ______________________________________                                        1               Positive E-field, E0                                          2               Negative E-field, -E0                                         3               Positive Reference, E1                                        4               Negative Reference, -E1                                       5               Positive Quadrature, E2                                       6               Negative Quadrature, -E2                                      ______________________________________                                    

The processor selects the larger of the absolute value of the positiveand negative values of each channel while storing the sign of eachvalue. The magnitude value of the Reference and Quadrature values is:

    Vmag=SQRT(E1.sup.2 +E2.sup.2)

The Reference and Quadrature values are then normalized:

    Vref=E1/Vmag

    Vquad=E2/Vmag

A principal angle is then computed as follows:

    Φ=arcsin(Vquad) for Vref>0.707 or

    Φ=arcsin(Vref)

The principal angle is the converted to the total angle using the signof the original values as follows:

    ______________________________________                                        Sign E0 Sign E1       Sign E2    Total angle                                  ______________________________________                                        +       +             +          θ = Φ                              +       -             +          θ = 180 - Φ                        +       -             -          θ = 180 + Φ                        +       +             -          θ = 360 - Φ                        -       -             -          θ = Φ                              -       +             -          θ = 180 - Φ                        -       +             +          θ = 180 + Φ                        -       -             +          θ = 360 - Φ                        ______________________________________                                    

Thus the direction or bearing of the initial discharge is determined.

The range is determined from the amplitude of the pulse. Since theinitial pulse has a known amplitude that falls off as a function ofdistance, the range can be directly determined as follows:

    Range=(RangeFactor1/E0)

A default value for RangeFactor1 is 20.0 based on empirical data, suchas from the circuit of FIG. 11. Other values of course can be used.

The E/H ratio is calculated as follows:

    E/H=E0/Vmag

The electronic circuit gains have been adjusted so that E/H isapproximately 1.0.

The E/H ratio for a lightning event is about 377, but circuitamplification gains can be adjusted so that in an exemplaryimplementation, the E/H ratio is about 1.

In step 412, the processor calculates a heading for an aircraft from acompass. The processor converts the bearing to north up using theheading in step 414. Bearing data is then formatted and sent to adisplay unit in step 416. The processor waits for one second untilwaiting for another trigger in step 418.

An altitude for a detected lightning discharge can be assigned afterdifferentiating between an IC and CG discharge. As shown in

FIG. 1, a default altitude of about 5 km can be assigned for a CGdischarge, and about 9 km for a IC discharge. The altitudes can befurther refined based upon the maturity and intensity of thethunderstorm based on lighting rates and rates of change.

As shown FIG. 16 in conjunction with FIGS. 4 and 5, a plot 63 of pulseamplitudes during a lightning stroke reveals a distributionapproximating a Rayleigh distribution. In particular, in between pulses58, corresponding to recoil streamers 56, during a flash 54 for example,a series of pulses are detected having durations of about a microsecondas is known in the art. Thousands of pulses are persistently radiatedduring a lightning event. The plot 63 of pulse amplitudes show that peakvalues are reached relatively rapidly followed by a more gradual declinein amplitude. It will be appreciated that pulses also occur after anintial pulse 62.

In a further embodiment, a lightning detection system having a singlesensor detects a series of about microsecond long pulses correspondingto a lightning stroke to obtain a peak value from a distribution ofpulse amplitudes which approximate a Rayleigh distribution. The peakamplitude value is used to determine a distance for the lightning event.In an exemplary embodiment, about twenty pulses are used to determine apeak value or an average value, however, other distributions havingother numbers of pulses are possible and contemplated. Thus, an intialpulse need not be detected. Return stroke pulses can be filtered out byrise time which typically are in the order of five microseconds.Alternatively, and providing a greater range, CG return strokes aredetected to determine a peak value from a Rayleigh distribution of pulseamplitudes corresponding to CG return strokes. Thus, more accurate eventlocations are obtainable in comparison with single sensor systemsutilizing an overall amplitude flash or ratios of amplitudes atdifferent frequencies.

FIG. 17 shows an alternative embodiment of a lightning detection systemincluding a non-weather marine radar and a lightning detector. Typicalmarine radars are sensitive to rainfall and provide a display of thedetected weather conditions on a display. A single station lightningdetector coupled to a weather radar adds lighting data to the display soareas of rainfall with lightning can be identified as thunderstorms. Inaddition, the lightning detector uses the radar rain data to increaselightning location accuracy.

In an exemplary embodiment, a lightning detection system 400 includes amarine radar 402 and a lightning detection unit 404 having a singlesensor to provide respective weather and lightning information to adisplay 406. An exemplary marine radar is a ship radar made by theMarine division of Raytheon. In an illustrative embodiment, the singlesensor first pulse ranging technique is used to obtain the distance tothe lightning and simple crossed loops would provide direction to thefirst pulse.

A further embodiment of a lightning detection system includes an opticallightning detector 408 shown in phantom. Systems for providing opticallightning detection are disclosed in U.S. Pat. No. 5,057,820 and in U.S.Pat. No. 5,396,220, both to Markson et al., both of which areincorporated herein by reference. In an exemplary embodiment, theoptical detector 408 is divided into eight 45° sectors. Detectedlightning is placed in one of three range bins, 0-5 nautical miles (NM),5-10 NM, and 10-30 NM base upon intensity of light detected from alightning discharge.

The range of an initial RF pulse is determined by the lightningdetection system from the amplitude of the pulse, typically having aduration of about one microsecond, and having a sharp rise. The range isdetermined from the amplitude of the detected pulse based on the factthat the signal intensity falls off with respect to distance as 1/range.The initial pulse, or pulses, measurements occur within the first 100meters of an initial leader channel over a period of about 1millisecond. By processing only an initial pulse, or pulses, systemprocessing complexity and capacity is reduced in comparison with systemsthat compute the positions of many strokes and pulses in each flash.Pulse information is formatted for the display 406 and integrated withweather data from the weather radar 402 for viewing by a user.

These and other examples of the concept of the aforedescribed inventionare intended by way of example and the actual scope of the invention isto be determined from the following claims.

What is claimed is:
 1. A lightning location detection system,comprising:at least one system of radio-frequency (RF) energy sensorsproviding an output signal indicative of detected RF energy; a filter inelectrical communication with said at least one system of RF energysensors, said filter receiving the output signal from said sensors andblocking low-frequency components of the output signal; and a signalprocessor in electrical communication with said filter, said signalprocessor determining the location of an initial lightning breakdownprocess responsive to the unblocked components of the sensor outputsignal corresponding to at least one of only a predetermined number ofinitial microsecond or shorter pulses produced by initial lightningbreakdown processes of initial leader strokes of lightning flashes. 2.The lightning location detection system according to claim 1, whereinsaid at least one system of RF energy sensors comprises three sensorspositioned at known locations such that said three sensors determine thelocation in two dimensions of the initial lightning breakdown processes.3. The lightning location detection system according to claim 2, whereinthe location of the initial lightning breakdown processes is determinedby measuring the difference between the time of arrival of RF energy ateach of said three sensors.
 4. The lightning location detection systemaccording to claim 3, wherein the difference between the arrival timesis processed using a least mean square algorithm.
 5. The lightninglocation detection system according to claim 1, wherein said at leastone system of RF energy sensors comprises four sensors positioned atknown locations for determining a location of the initial lightningbreakdown processes in three dimensions.
 6. The lightning locationdetection system according to claim 1, wherein said at least one systemof RF energy sensors comprises five sensors configured in a crossconfiguration.
 7. The lightning location detection system according toclaim 1, wherein said at least one system of RF energy sensors isdisposed in an aircraft.
 8. The lightning location detection systemaccording to claim 1, wherein said signal processor uses the amplitudeof the unblocked sensor output signals corresponding to at least one ofonly a predetermined number of initial microsecond or shorter pulses todetermine the distance to the initial lightning breakdown process. 9.The lightning detection system according to claim 8, wherein said signalprocessor determines a rate of initial lightning breakdown processes anduses the rate of initial lightning breakdown processes to predictmicrobursts.
 10. The lightning detection system according to claim 1,wherein said signal processor determines the polarity of the unblockedsensor output signals corresponding to said at least one of only apredetermined number of microsecond or shorter initial pulses and usesthe detected polarity to differentiate between intracloud initiallightning discharges and cloud to ground initial lightning discharges.11. The lightning location detection system according to claim 1,wherein said signal processor identifies non-lightning events by forminga ratio of the electric field strength to the magnetic field strengththe unblocked sensor output signals.
 12. The lightning locationdetection system according to claim 1, further comprising a GPS unit incommunication with said at least one system of RF energy sensors, saidGPS unit providing said at least one system of RF energy sensors with atime reference.
 13. The lightning location detection system according toclaim 1, wherein said at least one system of RF energy sensors detects arate of initial breakdown processes, said rate corresponding to stormintensity.
 14. The lightning location detection system according toclaim 9, wherein said signal processor uses intracloud lightning rateand intracloud lightning rate of change to predict the occurrence ofmicrobursts.
 15. The lighting location detection system according toclaim 1, wherein said at least one system of sensors comprises five ormore sensors configured in an array.
 16. The lightning detection systemof claim 8 wherein said signal processor determines the location of theinitial lightning breakdown processes responsive to the arithmetic meanof said sensor output signals.
 17. The lighting location of claim 1wherein said filter blocks components of the sensor output signal havinga frequency less than about 1.5 MHz.
 18. A lightning location detectionsystem, comprising:at least three RF energy sensors disposed in aspaced-apart relationship, each of said at least three sensorsincluding:a pulse detection circuit including a filter which blockslow-frequency components of received RF energy and passes high frequencycomponents of received RF energy for detecting the time of arrival ofthe leading edge of at least one of only a predetermined number ofmicrosecond or shorter initial pulses having predeterminedcharacteristics corresponding to initial lightning breakdown processesof an initial leader stroke of a lightning flash; a time circuit incommunication with said pulse detection circuit for associating a timeof arrival with passed high-frequency components of received RF energy;and a digital circuit in communication with said time circuit forcontrolling said time circuit, and in communication with said pulsedetection circuit for collecting data corresponding to passedhigh-frequency components of received RF energy; and a master station incommunication with each of said at least three RF energy sensors, saidmaster station collecting and processing the time of arrival associatedwith each detected time of the at least one of only a predeterminednumber of initial pulses for each of said at least three RF energysensors to determine a location of the initial lightning breakdownprocesses.
 19. A lighting location detection system, comprising:weatherradar for providing weather data; a lightning detection unit coupled tosaid weather radar, said lightning detection unit for detecting andprocessing at least one of only a predetermined number of microsecond orshorter initial pulses corresponding to initial lightning discharges ofinitial lightning strokes of a lightning flash, said lightning detectionunit providing locations for said initial lightning discharges, whereinsaid locations of said initial lightning discharges are integrated withsaid weather data provided by said weather data.